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Gforth Manual

3.18 General Loops

The endless loop is the most simple one:

: endless ( -- ) 0 begin dup . 1+ again ; endless

Terminate this loop by pressing Ctrl-C (in Gforth). begin does nothing at run-time, again jumps back to begin.

A loop with one exit at any place looks like this:

: log2 ( +n1 -- n2 ) \ logarithmus dualis of n1>0, rounded down to the next integer assert( dup 0> ) 2/ 0 begin over 0> while 1+ swap 2/ swap repeat nip ; 7 log2 . 8 log2 .

At run-time while consumes a flag; if it is 0, execution continues behind the repeat; if the flag is non-zero, execution continues behind the while. Repeat jumps back to begin, just like again.

In Forth there are many combinations/abbreviations, like 1+. However, 2/ is not one of them; it shifts it's argument right by one bit (arithmetic shift right):

-5 2 / . -5 2/ .

assert( is no standard word, but you can get it on systems other then Gforth by including `compat/assert.fs'. You can see what it does by trying

0 log2 .

Here's a loop with an exit at the end:

: log2 ( +n1 -- n2 ) \ logarithmus dualis of n1>0, rounded down to the next integer assert( dup 0 > ) -1 begin 1+ swap 2/ swap over 0 <= until nip ;

Until consumes a flag; if it is non-zero, execution continues at the begin, otherwise after the until.

Assignment:

Write a definition for computing the greatest common divisor.

Reference: 5.8.2 Simple Loops.